Which nutrient's digestion is initiated by saliva in the mouth?

The digestion of starch begins in the mouth, primarily facilitated by the enzyme amylase found in saliva. This enzyme specifically breaks down starch into simpler sugars, initiating the process of carbohydrate digestion before food even reaches the stomach. This is significant because it highlights the role of saliva in the digestive process, demonstrating that not all digestion occurs in the stomach or intestines.

While proteins and fats are important nutrients, their digestion does not start in the mouth; instead, protein digestion begins in the stomach with the action of pepsin, and fat digestion occurs primarily in the small intestine with the help of bile acids and lipases. Sugars, although they can be present in food, do not require a specific enzymatic action like amylase to begin their digestion in the mouth. Thus, given the options, starch is the correct nutrient whose digestion is initiated by saliva.